The use of thread-forming screws for securely coupling bodies is well known. This type of screw is inserted in an unthreaded bore or aperture and forms threads in the bore or aperture when a torque is applied. The screw structure is highly vibration-resistant for maintaining the coupling between the bodies even when subjected to considerable stress. Some self-threading screws include threads with multiple lobes around the circumference of the screw shank. The lobes, or undulations, further resist loosening of the screw.
Considerable torque is required in forming the threads on the screw's shank. The greater the required torque, the greater the required strength of the screw. The application of increased torque in the threading process as well as providing a screw comprised of materials of greater strength increase the cost of the end product. The goal is, therefore, to form the threads on the screw with a minimum torque. Where the screw is tapered, additional difficulty is encountered in providing the tapered portion with crested, or fully formed, threads. Fully crested threads are desirable because they facilitate insertion of the screw and also render it more difficult to remove the screw. However, providing the tapered end portion of the screw with fully crested threads generally requires a more complicated die configuration and increases the cost and complexity of thread-forming screw manufacture.
The present invention addresses the aforementioned limitations of the prior art by providing a thread-forming screw with a tri-roundular tapered end which facilitates insertion of the screw in a workpiece and maintains the screw securely in position therein.